1. Field of the Invention
The present invention pertains to apparatus and methods for drilling oil and or gas wells. More specifically, the present invention pertains to such a drilling unit and method suitable for drilling underwater wells.
2. Description of the Prior Art
There are several different types of rigs by which marine drilling is conducted. Of course one of the first developed was the fixed platform rig in which the legs or supports of the rig are permanently installed, penetrating the floor of the body of water in which the well is to be drilled. Such a rig is limited by water depth and does not provide the mobility and flexibility of the mobile or portable type drilling rig.
A later development was the submersible rig which is provided with a floating hull. This type of rig can be floated to a drilling site where the hull is filled with water ballast causing the hull to gradually sink until safety stabilized on the water body floor. After drilling is completed, the water ballast can be removed and the hull floated to the surface. This type of drilling rig is also limited by water depth. A recent example of a submersible type drilling rig is the one shown in U.S. Pat. No. 3,241,324. In an effort to overcome water depth limitations, various rigs have been developed which offer an extended height. See for example U.S. Pat. No. 2,938,354. To extend the height of this unit it is necessary to build it up by surmounting building block sections, one on top of the other.
In still later developments, drilling has been conducted from floating hull rigs. Although floating hull rigs are highly mobile, they are easily influenced by waves, winds, and other weather conditions, creating stability problems. Some of these problems have been alleviated by the development of "semi-submersible" rigs which derive their buoyancy from vertical columns or tanks, rather than from a conventional ship hull. Such a vessel is floated to a drilling site and partially submerged by flooding. The vertical columns or tanks have a relatively small exposed area at the waterline and consequently such a unit has a longer natural period in heave than does a hull type rig. The semi-submersible platform is, therefore, relatively less excited by waves and is usually quite stable in heave, pitch and roll.
A further development was the self-elevating platform, sometimes called "bootstrap" or "jack-up", rig in which a plurality of legs, usually three, are lowered from a floating platform into the water for penetration of the water body floor. The platform is then elevated on the legs a sufficient distance above the water surface. Although such rigs are highly mobile and stable when in place, they tend to be unstable when floating and when in transit from site to site and are limited by water depth. In areas of extreme weather conditions, the three or more legs of such rigs may have severe structural problems.
Although the different types of drilling units offer certain advantages, no one is completely suitable for every offshore drilling condition encountered. In an effort to incorporate some of the advantages of these units, various hybrid units have been developed which combine features of two or more of the basic type of drilling units. For example U.S. Pat. No 3,381,482 discloses a unit which combines the features of both submersible and semi-submersible type units. This unit has a submergible bottom supported lower portion and a semi-submersible upper portion which is extensible by ballast means.
U.S. Pat. No. 3,062,014 discloses a submersible type unit which is provided with a telescoping mast. After the entire unit is submerged for support on the floor of the body of water, the mast is extended upwardly to a desired position above the surface of the body of water. U.S. Pat. No. 3,456,447 discloses a drilling unit comprising a floatable tower and platform. The tower is floated to a selected site and submerged for support on the water body floor. Then the platform is floated around a portion of the tower extending above the surface of the water and raised to an elevated position thereon.